Composite articles are typically fabricated using a plurality of stacked preimpregnated fabric plies varying in size, shape and fiber matrix orientation. Typically, fabrication (prior to curing of the resin matrix) involves several independent process steps such as cutting individual plies to the desired shape, placing or stacking the plies in accordance with the desired fiber orientation, and forming each ply over a mold surface contour to ensure proper compaction of the plies during cure operations (i.e., without wrinkling, etc.).
Typically, these process steps are labor intensive. Generally, the ply is hand-placed into position, cut along a guide line to the desired shape, then removed from the top of the cutting apparatus (e.g. clicker die, gerber cutter, laser cutter). The precut ply is typically hand-carried to a mold station where the ply is laid up. At the mold station, the operator applies hand pressure to conform the ply to the mold surface contour tacking the ply to previously laid plies. Each ply normally includes a peel ply layer (plastic film) which is manually removed prior to stacking subsequent plies. The peel ply facilitates handling of the ply by protecting it from contamination and from sticking to handling equipment.
Recent efforts have focused on the automation of composite manufacturing. Progress has been made toward the development of automated cells for cutting, transfer and forming of preimpregnated plies to automate the lay-up operation.
For example, prepreg plies without the peel ply layer are transferred to a cutting head station where a numerically controlled head cuts the plies in a predetermined pattern. The cut plies are retrieved via a conveyor system and transferred to a mold surface by a robotic vacuum head. Once deposited, a flexible impervious membrane is used to shape the plies to a mold surface contour. The membrane is draped over the plies and sealed along its periphery to the mold. A vacuum is drawn in the cavity between the membrane and mold forcing the membrane to shape the plies to the mold surface contour.
Unfortunately in automated systems, the plies tend to stick to the cutting table, the conveyor belts, the vacuum head, flexible membrane, etc. due to the resin tack. A variety of techniques have been used to obviate this problem. For example, a vacuum apparatus may be used to separate the prepreg ply from the cutting table surface. However, the vacuum apparatus utilized is complex and subject to expensive breakdowns. In transfer equipment, positive pressure or extensible push pins have been utilized to release the prepreg plies from sticking to the vacuum heads. However, this has also proven complex.
The problem is exacerbated with automated forming equipment since the large surface area of a ply contacts the surface of a flexible membrane. One solution has been to heat the mold surface to increase the adhesion between the inner ply and the mold facilitating release of the prepreg from the flexible membrane. Other solutions have included applying a release agent to the ply to reduce tack. However, such substances can be a contaminant to the cured composite article.
Thus, although there has been a variety of solutions to the problem of handling tacky plies, none have proven satisfactory and there has been a continual search in this field of art for other solutions.